We have just had a serious occurrence of dusting of a vertical concrete surface, even though the wall was very carefully and thoroughly cured. Could the form oil we used be at fault?

How can the dusting surface be rectified?

Dusting on a vertical surface is likely to occur when the contractor has been using new wood for forms. Such wood may contain some sugar that will penetrate through the form oil and affect the surface of the concrete. This deleterious effect is normally confined to the first usage of the forms. The sugar is usually all gone the second time around.It has been found that a single wash of lime water applied to the new form material ahead of time neutralizes the action. A grout cleandown and sack rubbing (see item 71) will rectify the dusting. Another method is to apply a commercial concrete hardener of the zinc or magnesium fluosilicate type. Since there is no abrasion on the wall this should be a permanent cure.Dusting can also be due to such other chemical and physical factors, as:

  • Earth dust on forms. On walls and columns close to the ground, excessive amounts of ordinary ground dust build up and accumulate, especially in the corners of heavily oiled forms. Later, when the concrete is placed, this dust soaks up too much of the surface water so that the necessary cement hydrates cannot be formed. Especially bad in this respect is soil containing large amounts of those types of clays such as bentonite which are capable of absorbing up to five times their weight of water.
  • Use of porous timber. Certain soft woods such as brown (shingle type) cedar, soft pine and other soft woods, if used in forming, absorb a sufficient amount of water to prevent proper surface curing of the concrete.
  • Excessive heat. Sun temperatures above 100 degrees F beating down on wall forms, especially steel forms, will tend to induce the surface water to move inward to the cooler portion of the concrete, as well as upward, to impair proper surface curing.
  • Excessive water. Water from rains or surface flooding will tend to dilute or carry away sufficient amounts of the cementitious compounds from the bases of walls and piers before they can hydrate.
  • Chemical contamination. Muriatic acid and patented lime-solvent-type chemical cleaners are sometimes used to clean wood forms. The wood soaks up enough chemical to affect the concrete surface on the first reuse of the forms.